Objection 1. It seems that a degraded priest cannot consecrate this sacrament. For no one can perform this sacrament except he have the power of consecrating. But the priest "who has been degraded has no power of consecrating, although he has the power of baptizing" (App. Gratiani). Therefore it seems that a degraded priest cannot consecrate the Eucharist.
Objection 2. Further, he who gives can take away. But the bishop in ordaining gives to the priest the power of consecrating. Therefore he can take it away by degrading him.
Objection 3. Further, the priest, by degradation, loses either the power of consecrating, or the use of such power. But he does not lose merely the use, for thus the degraded one would lose no more than one excommunicated, who also lacks the use. Therefore it seems that he loses the power to consecrate, and in consequence that he cannot perform this sacrament.
On the contrary, Augustine (Contra Parmen. ii) proves that "apostates" from the faith "are not deprived of their Baptism," from the fact that "it is not restored to them when they return repentant; and therefore it is deemed that it cannot be lost." But in like fashion, if the degraded man be restored, he has not to be ordained over again. Consequently, he has not lost the power of consecrating, and so the degraded priest can perform this sacrament.
I answer that, The power of consecrating the Eucharist belongs to the character of the priestly order. But every character is indelible, because it is given with a kind of consecration, as was said above (63, 5), just as the consecrations of all other things are perpetual, and cannot be lost or repeated. Hence it is clear that the power of consecrating is not lost by degradation. For, again, Augustine says (Contra Parmen. ii): "Both are sacraments," namely Baptism and order, "and both are given to a man with a kind of consecration; the former, when he is baptized; the latter when he is ordained; and therefore it is not lawful for Catholics to repeat either of them." And thus it is evident that the degraded priest can perform this sacrament.
Reply to Objection 1. That Canon is speaking, not as by way of assertion, but by way of inquiry, as can be gleaned from the context.
Reply to Objection 2. The bishop gives the priestly power of order, not as though coming from himself, but instrumentally, as God's minister, and its effect cannot be taken away by man, according to Matthew 19:6: "What God hath joined together, let no man put asunder." And therefore the bishop cannot take this power away, just as neither can he who baptizes take away the baptismal character.
Reply to Objection 3. Excommunication is medicinal. And therefore the ministry of the priestly power is not taken away from the excommunicate, as it were, perpetually, but only for a time, that they may mend; but the exercise is withdrawn from the degraded, as though condemned perpetually.
Source: Summa Theologica (New Advent)